Self-immolative polymers are polymers that irreversibly disassemble into one or more compounds spontaneously or when triggered by a specific external stimulus or catalyst. Self-immolative polymers that decompose or depolymerize into small molecules or revert back to their monomer units can be used in applications such as time-release drug delivery, dry developing photoresists in the manufacture of integrated circuits or other similar devices, decomposable plastic items, or transient electronics. Self-immolative polymers are also sometimes referred to as sacrificial polymers. The transient nature of the device eliminates the concern of filling land-fills with long-lived garbage. Also, the transient nature of the device can be of high-value when recovery or detection of the device is not desired or possible. Devices which decompose by themselves or when triggered by an external stimulus prevent detection recovery and reverse engineering by competitors or hostile agents.
U.S. Pat. No. 9,496,229 discloses materials for transient electronic devices. The destruction or partial destruction of inorganic or hybrid inorganic/organic substrates is achieved by transforming the material into an inoperable state. Inorganic metals and semiconductors can be rendered inoperable by changing their oxidation state (i.e., oxidizing them) or dissolving them so that the solid-state properties are disrupted when the material is dissolved. U.S. Pat. No. 6,165,890 teaches that polymeric materials, such as poly(propylene carbonate) (PPC) can be transformed into gaseous products producing enclosed air cavities. The small molecule products produced upon polymer degradation can permeate through a wide variety of encapsulants. Both these patents describe inorganic and organic materials that are triggered into decomposition by chemical, thermal, or photo-triggers. However, the stimulus is applied to the whole material such that the structure is decomposed part-by-part or molecule-by-molecule. In the case of PPC, the temperature of the polymer must be raised to a value where the trigger or catalyst is effective—where the PPC is suitably energized so that once decomposition in initiated, complete disposal of the polymer can occur. In the case of inorganic materials, such as metals or semiconductors, enough reactant and energy of activation must be provided so that at least partial decomposition of the material is achieved, such as at least 20%. While these materials and methods of destruction may render a device inactive, they require a considerable amount of reactant be provided so that each layer or molecule of the device is treated or activated. In addition, the thermal activation must be at a minimum temperature, often over 100° C., for an extended period of time so that each chemical bond that needs to be broken has an opportunity to react. The time delay is sometimes an advantage as in the case of the slow release of gas from a buried air-cavity where excessive pressure build up would destroy the delicate overcoat structure as described in: Wu, X.; et al. Fabrication of Microchannels using Polynorbornene Photosensitive Sacrificial Materials. J. Electrochem. Soc. 2003, 150(9):H205-H213. In addition to the above mentioned problems, transient materials which require a large quantity of reactant or high temperatures or other excessive triggering conditions are not fail-safe. That is, if the reactant is depleted or the accelerating condition is inadequate, the transience process will terminate leaving the device vulnerable to capture, reverse-engineering, or simply defeat the intended transient purpose.
Thermodynamically unstable polymers have emerged in applications where a catalytic response to a small trigger is adequate to initiate the decomposition of a large quantity of material resulting in essentially full destruction. Low ceiling temperature polymers, which have a ceiling temperature (Tc) below, e.g., ambient, are especially valuable. Tc is the temperature where below it, the polymer state is favored and above Tc the monomer or depolymerized state is favored. Low Tc materials can be stable at temperatures above Tc if they are kinetically trapped in the polymer state. Kinetic trapping means that the mechanism of decomposition or depolymerization is slow, or essentially zero, unless catalyzed. When Tc is below the ambient temperature, only a single bond in a long polymer chain may be to be broken to overcome the kinetic trapping. This may lead to fail-safe decomposition of the device because the material is thermodynamically unstable at the target ambient temperature. It may also lead to vanishingly small amounts of a trigger, whether it is a chemical, thermal, photoactivated or other trigger.
While low Tc polymers are of interest, they can be difficult to control due to short shelf-life. In addition, it is difficult to synthesize nonphthalaldehyde-based copolymers and only aryl or ester copolymers of phthalaldehyde have been made (Kaitz et al., Macromolecules, 2014, 47:5509-5513). However, the ceiling temperature of glyoxylate-based polymers is too high; the molecular weight of the polymers is very low (less than 19.7 kDa); the rate of depolymerization is very slow, and the vapor pressure for some of the products is not adequate. There is also a lack of suitable triggering mechanisms, especially at low temperatures, such as e.g. 0° C., even though this may be above Tc.
What are thus needed are self-immolative polymers that have suitable mechanical properties and yet degrade/decompose completely upon exposure to a desired stimulus. Methods of making such polymers and articles comprising such polymers are also needed. The compositions, articles, and methods disclosed herein address these and other needs.